1. Field of the Invention
The present invention relates to a protection circuit used in a battery charger, a battery pack, etc., for secondary batteries, and more specifically to a technique to make possible to charge a secondary battery at a high voltage securing safety.
2. Description of the Related Art
A protection circuit for preventing overcharging and executing the charging management is provided in a battery charger and a battery pack of lithium ion secondary batteries. FIG. 4 depicts an example of a charging system including a charger 1 for a lithium ion secondary battery and a battery pack 2 that incorporates the lithium ion secondary battery. In FIG. 4, the charger 1 is provided with two P-type MOSFETs 121 and 122 that control the supply of a DC voltage of a voltage supplying circuit 11 and a protection circuit IC 13 that controls turning on and off of the MOSFETs 121 and 122 corresponding to the inter-terminal voltage of the battery pack 2.
The protection circuit IC 13 includes four terminals that are a positive input terminal 1311, a negative input terminal 1312, and two gate voltage output terminals 1313 and 1314 connected to a control circuit 131, and the IC 13 further includes the control circuit 131, a comparator 132, a comparing voltage generating circuit 133, and a reference voltage generating circuit 134.
The comparing voltage generating circuit 133 generates a comparing voltage VHC based on a reference voltage VOC supplied from the reference voltage generating circuit 134. A non-inverting input of the comparator 132 is applied with the comparing voltage VHC output from the comparing voltage generating circuit 133 and an inverting input of the comparator 132 is applied with a battery voltage VBATT of the lithium ion secondary battery 21. Actually, the inverting input of the comparator 132 is not input with the battery voltage VBATT having its full value, but input with a voltage having a value obtained by subtracting: a value of voltage drop caused by passing through the MOSFETs 121 and 122, etc.; from a value of the battery voltage VBATT.
An output voltage of the comparator 132 is input to the control circuit 131. The control circuit 131 controls an output voltage to the gate voltage output terminals 1313 and 1314 corresponding to the input voltage. For example, when the battery voltage VBATT and the comparing voltage VHC are in VBATT>VHC, the control circuit 131 turns off the MOSFET 121 and turns on the MOSFET 122, thereby stopping supply of the charging voltage to the lithium ion secondary battery 21. Even in this case, the lithium ion secondary battery 21 can discharge through a parasitic diode 1211 of the MOSFET 121.
In FIG. 4, the battery pack 2 is provided with two N-type MOSFETs 221 and 222 that control the supply of the charging voltage of the lithium ion secondary battery 21, and a protection circuit IC 23 that controls turning on and off of the MOSFETs 221 and 222 corresponding to the battery voltage of the lithium ion secondary battery 21. The protection circuit IC 23 includes four terminals of a positive input terminal 2311, a negative input terminal 2312, and two gate voltage output terminals 2313 and 2314 connected to a control circuit 231, and the IC 23 further includes the control circuit 231, a comparator 232, a comparing voltage generating circuit 233, and a reference voltage generating circuit 234.
The comparing voltage generating circuit 233 generates a comparing voltage VHB based on a reference voltage VOB supplied from the reference voltage generating circuit 234. A non-inverting input of the comparator 232 is input with a comparing voltage VHB and an inverting input thereof is input with the battery voltage VBATT of the lithium ion secondary battery 21. Actually, the inverting input of the comparator 232 is not input with the battery voltage VBATT having its full value, but input with a voltage having a value obtained by subtracting: a value of voltage drop caused by passing through the MOSFETs 221 and 222, etc.
An output voltage of the comparator 232 is input to the control circuit 231. The control circuit 231 controls an output voltage of the gate voltage output terminals 2313 and 2314 corresponding to the input voltage. For example, when the battery voltage VBATT and the comparing voltage VHB are in VBATT>VHB, the control circuit 231 turns off the MOSFET 221 and turns on the MOSFET 222, thereby stopping supply of the charging voltage to the lithium ion secondary battery 21. Even in this case, the lithium ion secondary battery 21 can discharge through parasitic diode 2211 of the MOSFET 221.
In the charging system configured as above, when the protection circuit IC 23 of the battery pack 2 has detected overcharging before the protection circuit IC 13 of the charger 1 detects the overcharging, a state may occur, where the charging voltage is supplied from the charger 1 to the lithium ion secondary battery 21 even when a current path in the charging direction is blocked in the battery pack 2. Therefore, it is common to set a relation between the comparing voltages VHC and VHB as VHC<VHB in order that the protection circuit IC 13 of the charger 1 operates before the protection circuit IC 23 of the battery pack does (see Japanese Patent Application Laid-Open Publication Nos. 2002-315215 and 2001-112182).
It is well known that the amount of charge (the energy density) held in the lithium ion secondary battery 21 can be increased with increasing a voltage at which the battery is charged. Therefore, the amount of charge of the lithium ion secondary battery 21 can be increased by setting the comparing voltages VHC and VHB to be as close as possible to the limit voltage, above which the battery is overcharged (hereinafter referred to as “charging limit voltage VL”).
However, considering the manufacture unevenness, temperature characteristics, etc., of the reference voltage generating circuits 134 and 234 and the comparing voltage generating circuits 133 and 233, there is a limit for bringing the comparing voltages VHC and VHB close to the charging limit voltage VL with the configuration of the above protection circuit. This will be described with reference to FIG. 5. For example, it is assumed that when the charging limit voltage VL is 4.5 V, each of the unevenness between the reference voltage generating circuits 134 and 234 and the unevenness between the comparing voltage generating circuits 133 and 233 is in the range of ±50 mV. In this case, the comparing voltage VHC has variation in the range of ±100 mV. Considering the relation between the comparing voltages VHC and VHB that is VHC<VHB, it is also assumed that a safe range of 50 mV is secured between the upper limit of the variation of the comparing voltage VHC and the lower limit of the variation of the comparing voltages VHD and another safe range of 50 mV is also secured between the comparing voltage VHB and the charging limit voltage VL. In this case, as shown in FIG. 5, the comparing voltage VHB of the battery pack 2 is 4.35 V and the comparing voltage VHC of the charger 1 is 4.1 V. Therefore, the comparing voltage VHC must be set at a value lower than the charging limit voltage VL by as much as 0.4 V.